Crank, lower quality methamphetamine not the higher quality crystam version but the stuff that is brown some times and burns like hell. There are 2 types of gack. One eye and two eye. One eye refers to the burning pain and tearing in the eye after snorting. Two eye gack makes both eyes burn and water from snorting it in one nostril.
"I did some gack last week and I still feel like shit!"
by PharmaPharmer December 30, 2004
Methamphetamine (crank, speed). Describes the gag reflex triggered by the extreme burning sensation of the nasal passages when the drug is snorted, and the very bitter taste as it drains down the back of the throat.
The character 'Pooh-Bear' in the movie "The Salton Sea" had to have his nose surgically removed because he snorted too much gack.
by Anonymous July 13, 2003
1.A term used for the Drug Crystal Meth Amphetamine

2A some what derogatory name for person under the influence of crystal meth.
1. "They just smoked a fat bowl of gack."

or Thats some killer Gack!"
2. "That guy is a total effin gack." or "Quit being such a gack, your going to get us busted!" or "Never bring that gack over to my house again!"
by L8Nighter March 02, 2011
The sound made by an individual who is in the process of sucking dick. Typically a gagging sound made from the throat.
Brenda gacks on that niggas dick because she has a terrible gag reflex.

That chicken head is always gacking choad in the park.
by Catsleeze September 23, 2013
A word or sound muttered/or written by Professors of Anatomy and Physiology when proper terminlogy is not used. Also used un situations of extreme exacerbation.
Student Answer: Gluteus Med.
Professor Commentary on test: GACK! This is not proper terminology.

"GACK!, be quiet now! Or I will leave!"
by Rojay November 30, 2006
Any amount of useless or unwanted stuff (usually regarding possesions)
I've got too much damn gack in my attic. Guess I'll throw something on ebay.
by mudpup April 05, 2006
Contraction for go back.
Would you like go g'ack home this weekend?
by Brian April 12, 2004
To get messed up in the process of putting the team on your back and still going out like a champ.
His night would have been better if he hadn't Gacked after the race.
by bww13 December 05, 2010

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